The invention relates to prostheses in general, and more particularly to improvements in knee joint prostheses. Still more particularly, the invention relates to improvements in knee joint prostheses of the type wherein a tibial component has a shank which is to be implanted into the tibia, a femoral component has a shank which is to be implanted into the femur, and the two components are coupled to each other by a hinge joint (hereinafter called hinge for short) defining a pivot axis which is disposed at a fixed distance from the two components. Such prostheses enable the femur to pivot relative to the tibia (and/or vice versa) in a predetermined plane (which is normal to the pivot axis) between a first position in which the two shanks make an oblique angle and a second position in which the two shanks extend from the pivot axis in substantially diametrically opposite directions.
Knee joint prostheses of the above outlined character are implanted when the tissue in the region of the knee Joint to be replaced is no longer capable of properly connecting the femur with the tibia. Once the substance at the knee joint of a patient has been diagnosed as being incapable of properly connecting the tibia and the femur, it is equally impossible to replace the natural knee Joint with an endoprosthesis of the type wherein the hinge which enables the tibia and the femur to pivot relative to each other is capable of permitting movements in the longitudinal direction of the tibia and/or femur. The stability of endoprosthesis of the just outlined character is achieved in that the two parts of the endoprosthesis are form-lockingly connected to each other. The damaged tissue at the knee Joint is incapable of stabilizing an endoprosthesis to prevent undue material stresses of the system and/or luxations by permitting a shifting of the hinge. Moreover, such movements would cause considerable pain to the patient and would render a luxated endoprosthesis useless for its intended purpose.
The above outlined problems are much less acute when the natural knee Joint is replaced with a fixed hinge type knee joint prosthesis because neither the tibial component nor the femoral component can move in the radial direction of the pivot axis which is defined by the hinge. Such complete form-locking engagement between the leaves of the hinge as well as between the leaves and the respective (femoral and tibial) components contributes significantly to the stability of the prosthesis, and the likelihood of damage to the material of the prosthesis is negligible or nil. The femur and the tibia can pivot relative to each other within preselected limits but are incapable of performing any other relative movements. It will be appreciated that a prosthesis wherein the pivot axis which is defined by the pintle of the hinge is disposed at a fixed distance from the femoral and tibial components is less versatile than certain other prostheses wherein the pivot axis can move longitudinally of the tibia and/or femur and/or vice versa. Therefore, the movements which such prosthesis permits are not identical with movements of a healthy knee joint but constitute a compromise between natural movements and those which are permissible or acceptable in view of the nature of damage or injury to the replaced natural knee joint. Thus, the tibia is not free to move from side to side but can merely turn about a single axis which is defined by the pintle of the hinge.
Heretofore known attempts to enhance the versatility of a knee joint prosthesis include the utilization of a hinge which permits the femur and the tibia to move relative to each other about a plurality of different axes. Such pronounced movability must be achieved without risking a loosening of the tibial and/or femoral component in the respective bone. In accordance with a presently known proposal, the artificial knee joint is to permit angular movements of the tibia and femur about a so-called compromise axis which is defined by a part attached to the femur. Extensive experiments with such prostheses indicate that a satisfactory orientation of the compromise pivot axis is yet to be found. If the axis of an existing prosthesis is not fixed, its orientation is far from resembling that which is defined by a natural knee joint and, in addition, a knee joint prosthesis with a non-fixed pivot axis is unreliable and does not permit the patient to perform movements which even remotely resemble those of a natural knee joint.
Another drawback of heretofore known knee joint prostheses is that they develop mechanical problems in the region of the hinge and/or at the loci of implantation of tibial and femoral components into the respective bones. In fact, the adverse mechanical influences are frequently so pronounced that it becomes necessary to remove the implanted prosthesis and to replace it with a different artificial knee joint. Implantation of a fresh knee joint prosthesis almost invariably necessitates extensive resection of bones which leads to substantial losses of bone material and causes complications in the course of subsequent operations.
Additional problems arise in connection with the implantation of knee joint prostheses. This holds especially true for proper positioning of threaded fasteners or other locating, retaining and/or positioning parts which must be anchored in a bone or which must pass through an accurately selected portion of a bone in order to properly engage complementary parts in the implanted prosthesis. Heretofore known proposals to accurately position material removing apparatus with reference to the bones to be drilled and/or similarly treated are far from satisfactory.